Bistable nematic liquid crystal device

ABSTRACT

A bistable nematic liquid crystal device cell is provided with a surface alignment grating on at least one cell wall and a surface treatment on the other wall. Such treatment may be a homeotropic alignment or a planar alignment with or without an alignment direction, and zero or a non zero pretilt. The surface profile on the monograting is asymmetric with its groove height to width selected to give approximately equal energy within the nematic material in its two allowed alignment arrangements. The monograting may be formed by a photolithographic process or by embossing of a plastics material. The cell is switched by dc pulses coupling to a flexoelectric coefficient in the material, or by use of a two frequency addressing scheme and a suitable two frequency material. Polarisers either side of the cell distinguish between the two switched states. The cell walls may be rigid or flexible, and are coated with electrode structures, e.g. in row and column format giving an x,y matrix of addressable pixels on the cell.

[0001] This invention relates to bistable nematic liquid crystaldevices.

[0002] Liquid crystal devices typically comprise a thin layer of aliquid crystal material contained between cell walls. Opticallytransparent electrode structures on the walls allow an electric field tobe applied across the layer causing a re-ordering of the liquid crystalmolecules.

[0003] There are three known types of liquid crystal material, nematic,cholesteric, and smectic each having a different molecular ordering. Thepresent invention concerns devices using nematic materials.

[0004] In order to provide displays with a large number of addressableelements it is common to make the electrodes as a series of rowelectrode on one wall and a series of column electrodes on the othercell wall. These form e.g. an x, y matrix of addressable elements orpixels and, for twisted nematic types of devices, are commonly addressedusing rms. addressing methods.

[0005] Twisted nematic and phase change type of liquid crystal devicesare switched to an ON state by application of a suitable voltage, andallowed to switch to an OFF state when the applied voltage falls below alower voltage level, i.e. these devices are monostable. For a twistednematic type of device (90° or 270° degree twist as in U.S. Pat. No.4,596,446), the number of elements that can be rms. addressed is limitedby the steepness of a device transmission vs voltage curve as details byAlt and Pleschko in IEEE Trans ED vol ED 21 1974 pages 146-155. One wayof improving the number of pixels is to incorporate thin filmtransistors adjacent each pixel; such displays are termed active matrixdisplays. An advantage of nematic type of devices is the relatively lowvoltage requirements. They are also mechanically stable and have widetemperature operating ranges. This allows construction of small andportable battery powered displays. Another way of addressing largedisplays is to use a bistable liquid crystal device. Ferroelectricliquid crystal displays can be made into bistable device with the use ofsmectic liquid crystal materials and suitable cell wall surfacealignment treatment. Such a device is a surface stabilised ferroelectricliquid crystal device (SSFELCDs) as described by:- L J Yu, H Lee, C SBak and M M Labes, Phys Rev Lett 36, 7, 388 (1976); R B Meyer, Mol CrystLiq Cryst. 40, 33 (1977); N A Clark and S T Lagerwall, Appl Phys Lett,36, 11, 899 (1980). One disadvantage of ferroelectric devices is therelatively large voltage needed to switch the material. This highvoltage makes small portable, battery powered displays expensive. Alsothese displays suffer from other problems such as lack of shockresistance, limited temperature range and also electrically induceddefects such as needles.

[0006] If bistable surface anchoring can be achieved using nematics thena display can be made which has the merits of both the above mentionedtechnologies but none of the problems.

[0007] It has already been shown by Durand et al that a nematic can beswitched between two alignment states via the use of chiral ions orflexoelectric coupling: A Charbi, R Barberi, G Durand and PMartinot-Largarde, Patent Application No WO 91/11747, (1991) “Bistableelectrochirally controlled liquid crystal optical device”, G Durand, RBarberi, M Giocondo, P Martinot-Largarde, Patent Application No WO92/00546 (1991) “Nematic liquid crystal display with surface bistabilitycontrolled by a flexoelectric effect”. These are summarised as follows:

[0008] In Patent Application No WO 91/11747 a device is described withthe following characteristics:

[0009] 1. The cell is made using two surfaces which have SiO coatings ofappropriate thickness and evaporation angle to allow two stable statesto exist on each surface. Furthermore the two states on a surface aredesigned to differ in azimuthal angle by 45° and the surfaces areoriented to differ in azimuthal angle by 45° and the surfaces areoriented such that each of the two resulting domains are untwisted.

[0010] 2. The cell (of 6 μm thickness) is filled with 5 CB doped with0.5% benzyl quininium bromide and 1.8% phenyl lactic acid. The former isan electrically positive chiral ion with left hand twist while thelatter is a negative chiral ion with a right hand twist. Theconcentrations ensure that the final mixture has a very long pitch sothat the states in the thin cell are uniform.

[0011] 3. Application of a 110 V dc pulse for 40 μs enabled switchingbetween the two states. A lower threshold is observed for longer pulsee.g. an 80 V threshold is observed for 300 μs pulses.

[0012] 4. Addition of suitably oriented polarisers caused one state toappear black while the other appears white with a contrast ratio ofabout 20.

[0013] 5. A variant device is also mentioned which causes a short pitchchiral ion mixture between monostable surfaces which possess differentzenithal anchoring energies. Switching between a 180° twisted state anda uniform state is observed in a 4 μm cell for pulses over 50 V.

[0014] In Patent Application WO 92/00546 a device is described with thefollowing characteristics:

[0015] The cell is made using two surfaces which have SiO coatings ofappropriate thickness and evaporation angle to allow two stable statesto exist on each surface. Furthermore the two states on a surface aredesigned to differ in azimuthal angle by 45° and the surfaces areoriented such that each of the two resulting domains are untwisted.

[0016] The surfaces are also oriented in such a way that the pretiltedstate on one surface lines up with the untilted state on the othersurface and vice versa. Hence when filled with 5 CB, the two states areseen as shown in FIGS. 7B and 7C.

[0017] Application of a 14 V dc pulse across a 1 μm cell for 100 μsallows switching between the states. The final state is dependent on thesign of the pulse due to its coupling to the flexoelectric polarisation.The same voltage threshold is observed for switching in both directions.

[0018] The surface used by Durand to obtain bistable alignment was athin layer of SiO evaporated at a precise oblique angle. However thismethod suffers the disadvantage that any deviation in the evaporationangle, layer thickness or indeed any of the deposition parameters islikely to produce a surface with only monostable alignment. This makesthe oblique evaporation technique unsuitable, or very difficult, forlarge area displays.

[0019] U.S. Pat. No. 4,333,708 describes a multistable liquid crystaldevice in which cell walls are profiled to provide an array of singularpoints. Such substrate configurations provide multistable configurationsof the director alignments because disclination must be moved to switchbetween stable configurations. Switching is achieved by application ofelectric fields.

[0020] Another bistable nematic device is described in GB.2,286,467-A.This uses accurately formed bigratings on at least one cell wall. Thebigrating permits liquid crystal molecules to adopt two differentangular aligned directions when suitable electrical signals are appliedto cell electrodes, e.g. dc coupling to flexoelectric polarisation asdescribed in Patent Application No. WO.92/00546. Since in the twosplayed state the director is quite close to being in the plane of thelayer, the coupling between director and flexoelectric component can besmall, which may hinder switching in some circumstances.

[0021] According to this invention the above disadvantages are overcomeby a surface treatment to at least one cell wall that permits nematicliquid crystal molecules to adopt either of two pretilt angles in thesame azimuthal plane. The cell can be electrically switched betweenthese two states to allow information display which can persist afterthe removal of power.

[0022] The term same azimuthal plane is explained as follows; let thewalls of a cell lie in the x,y plane, which means the normal to the cellwalls is the z axis. Two pretilt angles in the same azimuthal planemeans two different molecular positions in the same x,z plane.

[0023] According to this invention a bistable nematic liquid crystaldevice comprises;

[0024] two cell walls enclosing a layer of liquid crystal material;

[0025] electrode structures on both walls;

[0026] a surface alignment on the facing surfaces of both cell wallsproviding alignment to liquid crystal molecules;

[0027] means for distinguishing between switched states of the liquidcrystal material;

[0028] CHARACTERISED BY

[0029] a surface alignment grating on at least one cell wall thatpermits the liquid crystal molecules to adopt two different pretiltangles in the same azimuthal plane.;

[0030] the arrangement being such that two stable liquid crystalmolecular configurations can exist after suitable electrical signalshave been applied to the electrodes.

[0031] The grating may have a symmetric or an asymmetric groove profile.

[0032] The grating may have an asymmetric groove profile which willinduce a pretilt of less than 90°, e.g. 50° to 90°. An asymmetricprofile may be defined as a surface for which there does not exist avalue of h such that;

ψ_(x)(h−x)=ψ_(x)(h+x)  (1)

[0033] for all values of x, where ψ the function describing the surface.

[0034] The gratings may be applied to both cell walls and may be thesame or different shape on each wall. Furthermore the grating profilemay vary within each pixel area, and or in the inter pixel gaps betweenelectrodes. One or both cell walls may be coated with a surfactant suchas lethecin.

[0035] The liquid crystal material may be non twisted in one or bothstable molecular configurations. The cell walls may be formed of arelatively thick non flexible material such as a glass, or one or bothcells walls may be formed of a flexible material such as a thin layer ofglass or a plastic material flexible e.g. polyolefin or polypropylene. Aplastic cell wall may be embossed on its inner surface to provide agrating. Additionally, the embossing may provide small pillars (e.g. of1-3 μm height and 5-50 μm or more width) for assisting in correctspacing apart of the cell walls and also for a barrier to liquid crystalmaterial flow when the cell is flexed. Alternatively the pillars may beformed by the material of the alignment layers.

[0036] The grating may be a profiled layer of a photopolymer formed by aphotolithographic process e.g. M C Hutley, Diffraction Gratings(Academic Press, London 1982) p 95-125; and F Horn, Physics World, 33(March 1993). Alternatively, the bigrating may be formed by embossing; MT Gale, J Kane and K Knop, J App. Photo Eng, 4, 2, 41 (1978), or ruling;E G Loewen and R S Wiley, Proc SPIE, 88 (1987), or by transfer from acarrier layer.

[0037] The electrodes may be formed as a series of row and columnelectrodes arranged and an x,y matrix of addressable elements or displaypixels. Typically the electrodes are 200 μm wide spaced 20 μm apart.

[0038] Alternatively, the electrodes may be arranged in other displayformats e.g. r−θ matrix or 7 or 8 bar displays.

[0039] The invention will now be described, by way of example only withreference to the accompanying drawings of which;

[0040]FIG. 1 is a plan view of a matrix multiplexed addressed liquidcrystal display;

[0041]FIG. 2 is the cross section of the display of FIG. 1;

[0042]FIG. 3 shows a top view and a side view of the mask and exposuregeometry used to produce a grating surface.

[0043]FIG. 4 is a cross section of the liquid crystal directorconfiguration on the grating surface which leads to a higher pretilt.

[0044]FIG. 5 is a cross section of the liquid crystal directorconfiguration on the grating surface which leads to a lower pretilt.

[0045]FIG. 6 is the energy of the two pretilt configurations as afunction of groove depth to pitch ratio (h/w).

[0046]FIG. 7 shows a cross section of a cell configuration which allowsbistable switching between the two states.

[0047]FIG. 8 shows the transmission of the cell and the applied signalsas a function of time.

[0048]FIG. 9 shows an example multiplexing scheme for the bistabledevice.

[0049]FIG. 10 shows an alternative cell configuration for bistableswitching.

[0050]FIG. 11 shows a cell configuration for bistable switching betweena non-twisted and a twisted state.

[0051] The display in FIGS. 1, 2 comprises a liquid crystal cell 1formed by a layer 2 of nematic or long pitch cholestertc liquid crystalmaterial contained between glass walls 3, 4. A spacer ring 5 maintainsthe walls typically 1-6 μm apart. Additionally numerous beads of thesame dimensions may be dispersed within the liquid crystal to maintainan accurate wall spacing. Strip like row electrodes 6 e.g. of SnO₂ orITO (indium tin oxide) are formed on one wall 3 and similar columnelectrodes 7 are formed on the other wall 4. With m-row and n-columnelectrodes this forms an m×n matrix of addressable elements or pixels.Each pixel is formed by the intersection of a row and column electrode.

[0052] A row driver 8 supplies voltage to each row electrode 6.Similarly a column driver 9 supplies voltages to each column electrode7. Control of applied voltages is from a control logic 10 which receivespower from a voltage source 11 and timing from a clock 12.

[0053] Either side of the cell 1 are polarisers 13, 13′ arranged withtheir polarisation axis substantially crossed with respect to oneanother and at an angle of substantially 45° to the alignment directionsR, if any, on the adjacent wall 3, 4 as described later. Additionally anoptical compensation layer 17 of e.g. stretched polymer may be addedadjacent to the liquid crystal layer 2 between cell wall and polariser.

[0054] A partly reflecting mirror 16 may be arranged behind the cell 1together with a light source 15. These allow the display to be seen inreflection and lit from behind in dull ambient lighting. For atransmission device, the mirror 16 may be omitted.

[0055] Prior to assembly, at least one of the cell walls 3, 4 aretreated with alignment gratings to provide a bistable pretilt. The othersurface may be treated with either a planar (i.e. zero or a few degreesof pretilt with an alignment direction) or homeotropic monostablesurface, or a degenerate planar surface (i.e. a zero or few degrees ofpretilt with no alignment direction).

[0056] Finally the cell is filled with a nematic material which may bee.g. E7, ZLI2293 or TX2A (Merck).

[0057] An example method used to fabricate the grating surface will nowdescribed with reference to FIG. 3.

EXAMPLE 1

[0058] A piece of ITO coated glass to form the cell wall 3, 4 wascleaned with acetone and isopropanol and was then spin coated withphotoresist (Shipley 1805) at 3000 rpm for 30 seconds giving a coatingthickness of 0.55 μm. Softbaking was then carried out at 90° C. for 30minutes.

[0059] A contact exposure was then carried out on the coated wall 3, 4using a chrome mask 20 containing 0.5 μm lines 21 and 0.5 μm gaps 22(hence an overall pitch of 1 μm) as shown in FIG. 3. The exposure wascarried out at non-normal incidence, in this case an angle of 60° wasused. Mask 20 orientation is such that the groove direction issubstantially perpendicular to the to plane of incidence as shown inFIG. 3. Exposure in this geometry leads to an asymmetric intensitydistribution and therefore an asymmetric grating profile (see forexample B. J. Lin, J. Opt. Soc. Am., 62, 976 (1972)). Coated cell walls3, 4 were exposed to light from a mercury lamp (Osram Hg/100) with anintensity of 0.8 mW/cm² for a period of about 40 to 180 seconds asdetailed later.

[0060] After the exposure the coated cell wall 3, 4 was released fromthe mask 20 and developed in Shipley MF319 for 10 seconds followed by arinse in de-ionised water. This left the cell wall's surface patternedwith an asymmetric surface modulation forming the desired gratingprofile. The photoresist was then hardened by exposure to deep UVradiation (254 nm) followed by baking at 160° C. for 45 minutes. Thiswas done to ensure insolubility of the photoresist in the liquidcrystal. Finally the grating surface is treated with a solution of thesurfactant lecithin in order to induce a homeotropic boundary condition.

[0061] Finite element analysis has been carried out in order to predictthe molecular (more correctly the director) configuration of a freelayer of nematic material on such grating surfaces. The results areshown in FIGS. 4, 5 and 6 where the short lines represent liquid crystaldirector throughout the layer thickness, with the envelope of the shortlines at the bottom showing the grating profile. In this case thegrating surface has been described by the function; $\begin{matrix}{{y(x)} = {\frac{h}{2}{\sin ( {\frac{2\pi \quad x}{w} + {A\quad {\sin ( \frac{2\pi \quad x}{w} )}}} )}}} & (2)\end{matrix}$

[0062] where h is the groove depth, w is the pitch and A is an asymmetryfactor. In FIGS. 4 and 5, A=0.5 and h/w=0.6. In FIG. 4, the finiteelement grid has been allowed to relax from an initial director tilt of80°. In this case the configuration has relaxed to a pretilt of 89.5°.However, if the initial director tilt is set to 30° then the gridrelaxes to a pretilt of 23.0° as shown in FIG. 5. Therefore the nematicliquid crystal can adopt two different configurations depending onstarting conditions.

[0063] In practice a nematic liquid crystal material will relax towhichever of these two configuration has the lowest overall distortionenergy. FIG. 6 shows the total energy (arbitrary units) of the highpretilt (solid circles) and the low pretilt (empty circles) state versesthe groove depth to pitch ratio (h/w). For low h/w, the high pretiltstate has the lowest energy and so the nematic will adopt a high pretiltstate. Conversely for large h/w, the low pretilt state has the lowestenergy and so this state is formed. However when h/w=0.52, the stateshave the same energy and so either can exist without relaxing into theother. Therefore if a surface is fabricated at, or close to thiscondition, then bistability can be observed in the pretilt. Withreference to the above fabrication details, an exposure time of 80seconds was found to lead to a bistable surface. In this case thebistability is purely a function of the surface and does not rely on anyparticular cell geometry. In this sense it is distinct from prior artsuch as U.S. Pat. No. 4,333,708 (1982).

[0064] One suitable cell configuration to allow switching between thebistable states is shown in FIG. 7 which is a stylised cross section ofthe device in which a layer 2 of nematic liquid crystal material withpositive dielectric anisotropy is contained between a bistable gratingsurface 25 and a monostable homeotropic surface 26. The latter surface26 could, for example, be a flat photoresist surface coated withlecithin. Within this device liquid crystal molecules can exist in twostable states. In state (a) both surfaces 25, 26 are homeotropic whereasin (b) the grating surface 25 is in its low pretilt state leading to asplayed structure. For many nematic materials, a splay or benddeformation will lead to a macroscopic flexoelectric polarisation whichis represented by the vector P in FIG. 7. A dc pulse can couple to thispolarisation and depending on its sign will either favour or disfavourconfiguration (b).

[0065] With the device in state (a), the application of a positive pulsewill still cause fluctuations in the homeotropic structure despite thepositive dielectric anisotropy. These fluctuations are sufficient todrive the system over the energy barrier that separates the twoalignment states. At the end of the pulse the system will fall intostate (b) because the sign of the field couples favourably with theflexoelectric polarisation. With the system in state (b), a pulse of thenegative sign will once again disrupt the system but now it will relaxinto state (a) as its sign does not favour the formation of theflexoelectric polarisation. In its homeotropic state, the bistablesurface is tilted at slightly less than 90° (e.g. 89.5°). This issufficient to control the direction of splay obtained when the cellswitches into state (b).

[0066] One particular cell consisted of a layer of nematic ZLI2293(Merck) sandwiched between a bistable grating surface and a homeotropicflat surface. The cell thickness was 3 μm. Transmission was measuredthrough the cell during the application of dc pulses at room temperature(20° C.). The polariser and analyser 13, 13′ on each side of the cell 1were crossed with respect to each other and oriented at ±45° to thegrating grooves. In this set up, the two states in FIG. 7, (a) and (b),appear black and white respectively when addressed as follows.

[0067]FIG. 8 shows the applied voltage pulses (lower trace) and theoptical response (upper trace) as a function of time. Each pulse had apeak height of 55.0 volts and a duration of 3.3 ms. Pulse separation was300 ms. With the first application of a positive pulse, the transmissionchanges from dark to light indicating that the cell has switched fromstate FIG. 7(a) to state (b). A second positive pulse causes a transientchange in transmission due to the rms effect of coupling to the positivedielectric anisotropy causing a momentary switching of the bulk materialto state (a). However, in this case the cell does not latch at thesurface and so remains in state (b). The next pulse is negative in signand so switches the cell from state (b) to state (a). Finally a secondnegative pulse leaves the cell in state (a). This experiment shows thatthe cell does not change state on each pulse unless it is of the correctsign. Thus it proves that the system is bistable and that the finalstate can be reliably selected by the sign of the applied pulse.

[0068] The switching occurs across a wide temperature range. As thetemperature is increased the voltage required for switching falls. Forexample at 30° C., a voltage of 44.8 V is required for bistableswitching whereas at 50° C. the voltage in only 28.8 V. Similarly, for afixed voltage the required pulse length for latching decreases withtemperature.

[0069] After this data was taken, the cell was dismantled and thegrating surface as characterised by AFM (atomic force microscopy). Anasymmetric modulation was confirmed which was fitted by equation 2 togive a pitch of 1 μm, a groove depth of 0.425 μm (h/w=0.425) and anasymmetry factor of A=0.5. In comparison to the results in FIG. 6, thisgrating has its bistable regime at a lower value of h/w (0.425 comparedto 0.52). However equation 2 was not a precise fit to the AFM data dueto the real surface possessing steeper facet angles which require theaddition of higher harmonics in the description. Other effects such asAFM tip radius also need to be considered for a more accuratecomparison. Thus it can be concluded that the measured surfacemodulation is similar to the predicted regime for bistability.

[0070] The successful switching of a single pixel allows the design of asuitable multiplexing method for the selection of several adjacentpixels. FIG. 9 shows one particular example of such a scheme. As shownpixels in four consecutive rows R1, R2, R3, R4 in one column are to beswitched. Two possible alignment states may be arbitrarily defined as ONand OFF states. Rows R1 and R4 are to switched to an ON state, rows R2and R3 are in the OFF state. Strobe pulses of +V_(s) for three timeslots followed by −V_(s) for 3 time slots (ts) are applied to each rowin turn. A data waveform is applied to the column as shown and comprisesa −V_(d) for 1 ts followed by a +V_(d) for 1 ts for and ON pixel, and−V_(d) for 1 ts followed by +V_(d) for 1 ts for and OFF pixel. Nowconsidering one particular pixel at A. The resultant waveform consistsof large positive and negative pulses which disrupt the nematicorientation and raises its energy up to the barrier that separates thetwo bistable surface states. In this field applied condition, the liquidcrystal molecules align along the electric field as in conventionalmonostable nematic devices, and as shown in FIG. 7a. These large ‘reset’pulses of opposite polarity are followed immediately by a smaller pulsewhich is still large enough to dictate the final selection state of thepixel during the relaxation of the orientation. Electrical balance isachieved by a small pulse of polarity opposite to the switching pulseand preceding the two large pulses. Alternatively, polarity inversion inadjacent display address time may be used.

[0071] The above bistable device achieves final state selection byvirtue of the flexoelectric polarisation in one state. Therefore thisconfiguration must contain splay. In the experimental example only onesurface is allowed to switch but working devices can also be made inwhich both surfaces switch. The only remaining constraint is that thelow pretilt states on each surface should differ in value so that afinite splay remains. However even if the low pretilt states are equal,the cell can still be switched if it contains a two frequency nematicmaterial, that is a material whose dielectric anisotropy is positive atlow frequencies and negative at high frequencies. An example of such amaterial is TX2A (Merck) which has a crossover frequency of 6 kHz. FIG.9 shows a cross section of this configuration. With the cell in state(a), the application of a high frequency signal drives the bulk of thenematic to a low pretilt. The surfaces follow and so the cell switchesto state (b). Conversely a low frequency signal will drive the nematicto a high pretilt and so the cell will switch to state (a).

EXAMPLE 2

[0072] A second example of a bistable device is now described. A pieceof ITO coated glass to form the cell wall was cleaned with acetone andisopropanol and was then spin coated with photoresist (Shipley 1813) at3000 rpm for 30 seconds giving a coating thickness of 1.5 μm. Softbakingwas then carried out at 90° C. for 30 minutes.

[0073] A contact exposure was then carried out using a chrome maskcontaining 0.5 μm lines and 0.5 μm gaps (hence an overall pitch of 1μm). In this example the exposure was carried out at normal incidence.Exposure in this geometry leads to a symmetric intensity distributionand therefore a symmetric grating profile. Samples were exposed to lightfrom a mercury lamp (Osram Hg/100) with an intensity of 0.8 mW/cm².

[0074] After the exposure the sample was released from the mask anddeveloped in Shipley MF319 for 20 seconds followed by a rinse inde-ionised water. This left the sample patterned with a symmetricsurface modulation. The photoresist was then hardened by exposure todeep UV radiation (254 nm) followed by baking at 160° C. for 45 minutes.This was done to ensure insolubility of the photoresist in the liquidcrystal. Finally the grating surface is treated with a solution of achrome complex surfactant in order to induce a homeotropic boundarycondition.

[0075] One particular surface was made using the above method with anexposure time of 360 s. AFM analysis on this grating showed it to have asymmetric profile with a pitch of 1 μm and a depth of 1.2 μm. Thissurface was constructed opposite a flat homeotropic surface to form acell with a thickness of 2.0 μm. The cell was filled with the nematicmaterial E7 (Merck) in the isotropic phase followed by cooling to roomtemperature. Microscopic observation revealed a mixture of both bistablestates, shown as (a) and (b) in FIG. 7.

[0076] The cell was oriented between crossed polarisers so that thegroove direction was at 45° to the polariser directions. Thus state (a)was the bright state while state (b) was the dark state. Monopolarpulses of alternating sign were then applied to the cell. The pulselength was set to 5.4 ms with a 1 s pulse separation. Full switchingoccurred between state (a) and (b) when the peak voltage of the appliedpulses was increased to 20.3 V. Pairs of pulses were also applied to thecell in a similar manner to the data shown in FIG. 8. Once again onlythe first pulse changed the state of the system while the second pulseonly induced a non-latching transient response. In this case the opticalresponse times were also measured. The 10%-90% response time forswitching from (a) to (b) was 8.0 ms while the response time forswitching from (b) to (a) was 1.2 ms. Further analysis of this cellrevealed that the bistable states (a) and (b) induced pretilts of 90°and 0° respectively on the grating surface. Thus this sample hasdemonstrated the maximum possible change in pretilt.

[0077] The optics of the configurations shown in FIG. 7 and 10 isoptimised when the cell thickness d is given by:- $\begin{matrix}{d = \frac{\lambda}{2\Delta \quad n_{av}}} & (3)\end{matrix}$

[0078] where λ is the operating wavelength and Δn_(av) is the averagevalue of the in-plane component (parallel to the cell walls) of thenematic birefringence. Δn_(av) will be larger for the configurationshown in FIG. 10 compared to FIG. 7, hence the cell thickness can beless and therefore the optical switching speed will be larger. Howeverthe use of a two frequency nematic limits the choice of availablematerials, also leads to a more complex addressing scheme, but may allowlower voltage operation.

EXAMPLE 3

[0079] The bistable grating surface can also be constructed opposite aplanar surface. One such cell consisted of a grating with the sameprofile to that described in example 2. This was constructed opposite arubbed polymer surface formed using a layer of PI32 polyimide (CibaGeigy). The rubbing direction on the polyimide surface was set parallelto the grating groove direction on the grating surface. The cell gap wasset to 2.5 μm and nematic E7 was used to fill the cell. Cooling to roomtemperature after filling revealed two states which are shownschematically in FIG. 11. This Figure differs from FIG. 7 in that thegroove direction on the bistable surface is now in the plane of the page(in an x,y plane). Thus the 90° pretilt state on the grating forms thehybrid structure shown in (a′) while the 0° pretilt state on the gratingforms the twisted structure shown in (b′). To achieve optical contrastbetween the states, the cell was placed in-between crossed polarisers13, 13′ oriented so that the grating grooves (and rubbing direction)were parallel to one polariser, although the polarisers may be rotatedto optimise contrast in the two switch states. Thus state (b′) was thebright state while state (a′) was the dark state. Using 5.3 ms monopolarpulses, switching between (a′) and (b′) occurred at a peak voltage of56.7 V. The optical response times were 110 ms for switching from (a′)to (b′) and 1.4 ms for switching from (b) to (a′).

[0080] The bright state (b′) has a bulk twist of 90°. As withconventional TN structures, the maximum transmission is obtained when Nis an integer where (C. H. Gooch and H. A. Tarry, J. Phys. D: Appl.Phys., 8 1575 (1975));

N={square root}(Δnd/λ)²+0.25  (4)

[0081] where Δn is the nematic birefringence, d is the cell gap and λ isthe operating wavelength. Therefore a bistable device using E7 (Δn=0.22)with an operating wavelength of 530 nm and N=1 will have a cell gap of2.1 μm.

[0082] In comparison the configuration described in example 2 has anoptimum thickness given by equation 3. For that example, Δn_(av) is Δn/2therefore equation 3 gives a thickness of 1.2 μm. Thus the bistabledevice without twist will always possess optimum optics at a thinnercell gap and will therefore switch at lower voltages with a shorteroptical response time.

[0083] A cholesteric dopant (eg <1% of CB15 Merck) may be added toprevent twist disclinations. Alternatively these disclinations may beprevented by arranging the groove directions non parallel to the rubbingalignment directions, eg about 5° adjustment.

[0084] The grating surfaces for these devices can be fabricated using avariety of techniques as listed earlier. The homeotropic treatment canbe any surfactant which has good adhesion to the grating surface. Thistreatment should also lead to an unpinned alignment. That is, analignment which favours a particular nematic orientation withoutinducing rigid positional ordering of the nematic on the surface.

[0085] As seen from the above analysis, the grating modulation has topossess a certain h/w for a given asymmetry for bistability to exist.The absolute scale of the modulation is limited by other factors. If thegroove depth and pitch are too large then diffractive effects will besignificant and lead to loss of device throughput. Furthermore if thegroove depth is similar to the cell thickness then the proximity of thegroove peaks to the opposite flat surface may inhibit bistableswitching. If two gratings are required as in the device shown in FIG.10 then a large groove depth compared to cell thickness would inevitablylead to switching which depends on the phase of the two modulations.This would severely complicate the device manufacturing process.

[0086] Problems also exist if the groove depth and pitch are too small.For a constant h/w, as the pitch becomes smaller the energy density ofthe bulk distortion at the surface becomes larger. Eventually thisenergy is similar to the local anchoring energy of the nematic on thesurface. Thus the structures shown in FIGS. 4 and 5 (which assume aninfinite anchoring energy) would no longer be obtained and bistabilitywould inevitably be lost. Typical values of h and w are, about 0.5 μmand 1.0 μm in a range of about 0.1 to 10 μm and 0.05 to 5 μmrespectively.

[0087] Small amounts e.g. 1-5% of a dichroic dye may be incorporatedinto the liquid crystal material. This may be used with or without apolariser, to provide colour, to improve contrast, or to operate as aguest host type device; e.g. the material D124 in E63 (Merck). Thepolarser(s) of the device (with or without a dye) may be rotated tooptimise contrast between the two switched states of the device.

1. A bistable nematic liquid crystal device comprising two cell wallsenclosing a layer of liquid crystal material; electrode structures onboth walls; a surface alignment on the facing surfaces of both cellwalls providing alignment to liquid crystal molecules; means fordistinguishing between switched states of the liquid crystal material;CHARACTERISED BY a surface alignment grating on at least one cell wallthat permits the liquid crystal molecules to adopt two different pretiltangles in the same azimuthal plane; the arrangement being such that twostable liquid crystal molecular configuration can exist after suitableelectrical signals have been applied to the electrodes.
 2. The device ofclaim 1 wherein the grating comprises a single asymmetric modulationwhose groove depth and pitch ratio provides bistable alignment.
 3. Thedevice of claim 1 wherein the grating comprises a single symmetricmodulation whose groove depth and pitch ratio provides bistablealignment.
 4. The device of claim 1 wherein the grating material inducesa homeotropic orientation of the liquid crystal director with respect tothe local surface direction.
 5. The device of claim 1 wherein thegrating surface is treated with a surfactant in order to induce ahomeotropic orientation of the liquid crystal director with respect tothe local surface direction.
 6. The device of claim 1 wherein one cellwall has a bistable grating surface and the other cell wall has a flatsurface which induces a homeotropic alignment.
 7. The device of claim 1wherein one cell wall has a bistable grating surface and the other cellwall has a flat surface which induces a planar alignment with or withouta preferred alignment direction.
 8. The device of claim 7 wherein theplanar alignment provides a surface pretilt between 0° and 60°.
 9. Thedevice of claim 1 wherein both cell walls are bistable grating surfaces.10. The device of claim 1 wherein the liquid crystal material has apositive dielectric anisotropy.
 11. The device of claim 1 and furtherincluding means for applying unidirectional voltage pulses whereby thebistable state is selected by a coupling between the applied field andthe flexoelectric polarisation present in a bent or splayed liquidcrystal material.
 12. The device of claim 1 and further including meansfor applying unidirectional signals at two different frequencies andwherein the nematic material is two-frequency addressable liquid crystalmaterial.
 13. The device of claim 1 wherein the grating groove profilevaries within each pixel area and or between adjacent pixels.
 14. Thedevice of claim 1 wherein the electrodes are formed as a series of rowand column electrodes arranged in an x, y matrix of addressable elementsor display pixels.
 15. The device of claim 1 wherein the means fordistinguishing between switched states includes a dichroic dye in theliquid crystal material.
 16. The device of claim 1 wherein the means fordistinguishing between switched states includes at least one polariser.